Buy a blind labelled blackout, fit it properly, and there is still a bright outline around the window at six in the morning. This is the single most common complaint about blackout blinds, and almost none of it is a manufacturing fault.
The fabric is doing its job. The opening is the problem.
Fabric Darkness and Light Leakage Are Two Different Things
A blackout fabric is one that light cannot pass through. That is a property of the cloth, and a good one will block essentially all of it across its own surface. What the label does not describe is the route light takes around the cloth, and in a normal window that route is generous.
Think of it as the difference between a wall and a door. The material matters, but the frame is what decides whether the room is dark. A blind is only ever as good as its worst gap, which is why blackout blinds with side tracks behave so differently from an ordinary roller in the same recess.
The Three Gaps, in Order of How Much They Cost You
Start with the sides. Even a well measured roller blind sits a few millimetres clear of the reveal so it does not catch as it travels, and that clearance becomes two vertical strips of daylight on the wall. On a bright morning these are usually the brightest thing in the room.
Then the top. On a standard roller, fabric leaves the tube on one side, which means there is an open channel behind the headrail. Light drops straight through it.
Finally the bottom. Sills are rarely level, and a rigid bottom bar meeting an uneven surface leaves a wedge of light at one end. Older properties are worse for this, which is not a reason to avoid old properties.
What Actually Closes Them
Side channels are the fix that people notice most. Fitting a track down each reveal captures the fabric edge and removes the vertical leak. A boxed or cassette headrail deals with the top. A bottom bar with a soft seal, or simply a blind measured to the shortest point of the sill rather than the longest, handles the third.
Fitted together, that is a system rather than a single product, and it is worth understanding before you replace a blind you already own with a slightly darker version of the same thing.
Fitting Without Drilling Into the Frame
There is a complication in newer homes. Sealed double glazed units sit in frames you should not be putting screws into, partly because of the warranty and partly because you can breach the seal. Tenants have the same problem for a different reason.
This is where clip-in systems earn their keep. The no-drill range uses brackets that grip the frame or the beading rather than fixings through it, so the blind sits inside the recess, which is exactly where a blackout blind needs to be, without a single hole.
When to Change the Blind and When to Change the Approach
If your current blind is dark in the middle and bright at the edges, buying a darker fabric is money spent on the part that already works. Address the perimeter instead.
If the room needs both darkness and insulation, a no-drill honeycomb blackout blind is worth a look, because the cell structure blocks light through its thickness and holds a layer of air against the glass at the same time.
And if the room is not a bedroom, ask whether you need blackout at all. Full darkness in a room you use during the day is a purchase most people end up working around.
